Gear device

ABSTRACT

A gear device includes a first gear, a second gear, a third gear, a housing, and a cover. The second gear is offset eccentrically when the first gear is rotated. The third gear is rotated at a speed slower than a rotation speed of the first gear when the second gear is offset eccentrically. The housing accommodates the first gear, the second gear, and the third gear. The cover is attached to the housing. The cover includes a deformation resistance portion that limits deformation of the cover in an axial direction of the third gear.

BACKGROUND 1. Field

The present disclosure relates to a gear device.

2. Description of Related Art

Japanese Laid-Open Patent Publication No. 2019-62679 describes a gear device for a seat lifter.

As shown in FIG. 14 , a gear device 70 of the above publication includes a worm gear (worm wheel) 75, which serves as a first gear coupled to an eccentric shaft 74, and an external gear 72, which serves as a second gear offset eccentrically by rotation of the worm gear (worm wheel) 75. The eccentric offsetting of the external gear 72 rotates an internal gear 73, serving as a third gear, at a slower speed than the worm gear (worm wheel) 75. The gear device 70 includes a stopper plate 76 that permits linear movement of the external gear 72 and restricts rotational movement of the external gear 72.

The gear device 70 includes a housing 77 that accommodates the stopper plate 76 in a state permitting linear movement of the stopper plate 76 and restricting rotational movement of the stopper plate 76. A cover 78 is attached to the housing 77 in a state accommodating the worm gear (worm wheel) 71, the external gear 72, and the internal gear 73 in addition to an urging member 79.

As shown in FIG. 14 , in the gear device 70 of the above publication, the repulsion force of the urging member 79 produces an upward urging force. Further, the components in the housing that cooperate with one another produce upward stress. Such urging force and stress may upwardly move the outer circumferential portion of the internal gear 73 and deform the cover 78 in the thickness direction.

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In one general aspect, a gear device includes a first gear, a second gear offset eccentrically when the first gear is rotated, a third gear rotated at a speed slower than a rotation speed of the first gear when the second gear is offset eccentrically, a housing that accommodates the first gear, the second gear, and the third gear, and a cover attached to the housing. The cover includes a deformation resistance portion that limits deformation of the cover in an axial direction of the third gear.

This configuration limits deformation of the cover in the thickness direction.

Other features and aspects will be apparent from the following detailed description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a seat.

FIG. 2 is a perspective view of a drive device.

FIG. 3 is an exploded perspective view of the drive device.

FIG. 4 is a cross-sectional view of the drive device.

FIG. 5 is a perspective view of a housing and a stopper plate.

FIG. 6 is a plan view of the housing and the stopper plate.

FIG. 7 is an enlarged view of an accommodation pit of a housing.

FIG. 8 is an enlarged view of the accommodation pit of the housing in a state accommodating an arm of the stopper plate.

FIG. 9 is a perspective view of the stopper plate and a second gear.

FIG. 10 is a plan view of the stopper plate and the second gear.

FIG. 11 is a perspective view showing a rear side of the second gear.

FIG. 12 is a side view of the second gear.

FIG. 13 is a plan view of an engagement claw.

FIG. 14 is a cross-sectional view of a prior art gear device.

Throughout the drawings and the detailed description, the same reference numerals refer to the same elements. The drawings may not be to scale, and the relative size, proportions, and depiction of elements in the drawings may be exaggerated for clarity, illustration, and convenience.

DETAILED DESCRIPTION

This description provides a comprehensive understanding of the methods, apparatuses, and/or systems described. Modifications and equivalents of the methods, apparatuses, and/or systems described are apparent to one of ordinary skill in the art. Sequences of operations are exemplary, and may be changed as apparent to one of ordinary skill in the art, with the exception of operations necessarily occurring in a certain order. Descriptions of functions and constructions that are well known to one of ordinary skill in the art may be omitted.

Exemplary embodiments may have different forms, and are not limited to the examples described. However, the examples described are thorough and complete, and convey the full scope of the disclosure to one of ordinary skill in the art.

In this specification, “at least one of A and B” should be understood to mean “only A, only B, or both A and B.”

With reference to FIGS. 1 to 13 , a gear device 32 will now be described.

Seat Lifter

FIG. 1 is a side view of a seat 11. The seat 11 is arranged in a vehicle such as a four-wheel automobile. Slide rails 12 are arranged on a floor 1 of the vehicle. Each slide rail 12 includes two upper rails 21 and two lower rails 22. The lower rails 22 are coupled to the floor 1 of the vehicle. The upper rails 21 are coupled to the lower rails 22 in a manner slidable in the front and rear direction.

Brackets 23 are coupled to each upper rail 21 near the front and rear ends. A front link 26 is coupled by a pin 24 to each front bracket 23. A rear link 27 is coupled by a pin 24 to each rear bracket 23. Thus, two front links 26 and two rear links 27 are coupled by the brackets 23 to the two upper rails 21. Each front link 26 and each rear link 27 is pivotally coupled to the corresponding bracket 23.

The front links 26 and the rear links 27 each include an end that is opposite to the end coupled to the corresponding bracket 23 and coupled by a rod 28 to a cushion frame 25. The front links 26 and the rear links 27 are pivotally coupled to the cushion frame 25.

The brackets 23, the front links 26, the rear links 27, and the cushion frame 25 form a four-joint pivoting mechanism.

One of the two rear links 27 includes a gear 27 a. The gear 27 a includes teeth arranged in an arcuate manner about the corresponding rod 28.

A drive device 29 is coupled to the cushion frame 25. The drive device 29 incudes a pinion 29 a. The pinion 29 a meshes with the teeth of the gear 27 a on the rear link 27 The drive device 29 is driven to rotate the pinion 29 a. Rotation of the pinion 29 a changes the position of the pinion 29 a relative to the gear 27 a. This changes the distance between the pin 24, which is fixed to the rear link 27, and the pinion 29 a. When the distance changes between the pin 24, which is fixed to the rear link 27, and the pinion 29 a, the four-joint pivoting mechanism acts to pivot the rear link 27 about the pin 24. This raises or lowers the cushion frame 25 relative to the brackets 23.

The brackets 23, the front links 26, the rear links 27, the cushion frame 25, and the drive device 29 form the seat lifter. The drive device 29 may be referred to as a gear box motor.

Drive Device 29

As shown in FIGS. 2 and 3 , the drive device 29 includes a motor 31 and a gear device 32. The gear device 32 reduces the speed of the rotation generated by the motor 31 to rotate the pinion 29 a. Thus, the gear device 32 may be referred to as a reduction gear. FIG. 2 does not accurately show how many teeth the pinion 29 a has. Likewise, other drawings do not accurately show the teeth of gears.

As shown in FIG. 3 , the motor 31 includes a case 31 a and components accommodated in the case 31 a. The components include, for example, a motor shaft, a stator, a rotor, coils, and magnets (none shown). The motor 31 is driven by electric power and rotates the motor shaft about a rotation axis Ax1.

Gear Device 32

As shown in FIGS. 3 and 4 , the gear device 32 includes a rotation transmission mechanism 35, a hollow housing 36 having a closed bottom end, and a cover 37.

The rotation transmission mechanism 35 is accommodated in the housing 36. The rotation transmission mechanism 35 includes a first reduction unit 41 serving as a reduction mechanism, a second reduction unit 42 also serving as a reduction mechanism, and an output shaft 43. The drive force of the motor 31 is transmitted via the first reduction unit 41 to the second reduction unit 42, and further from the second reduction unit 42 to the output shaft 43.

The output shaft 43 is supported by the housing 36 and the cover 37 so as to be rotatable about a rotation axis Ax2. The rotation axis Ax2, which is the center axis of the output shaft 43, extends in a direction intersecting the direction in which the rotation axis Ax1 extends.

The pinion 29 a of the drive device 29 is arranged on the output shaft 43. The pinion 29 a projects out of the housing 36. The pinion 29 a is rotatable about the rotation axis Ax2 integrally with the output shaft 43.

As shown in FIGS. 3 and 4 , the direction in which the rotation axis Ax2 extends is referred to as the up-down direction DZ, the direction in which arms 63 b of a stopper plate 63, which will be described later, is referred to as the width direction DX, and the direction in which recesses 63 c of the stopper plate 63 are arranged next to each other is referred to as the front-rear direction DY

Rotation Transmission Mechanism 35

As shown in FIG. 3 , the first reduction unit 41 includes a worm 51 and a worm wheel 52, which serves as a gear portion. The worm 51 is coupled to the motor shaft of the motor 31 and rotated integrally with the motor shaft about the rotation axis Ax1.

As shown in FIGS. 3 and 4 , the worm wheel 52 is supported by the housing 36 and the output shaft 43 so as to be rotatable about the rotation axis Ax2. The worm wheel 52, the center of which is the rotation axis Ax2, is disc-shaped and supported in the housing 36 by an urging member 39, such as a Belleville spring.

In the first reduction unit 41, when the driving force of the motor 31 rotates the worm 51 about the rotation axis Ax1, the worm wheel 52, which is meshed with the worm 51, is rotated about the rotation axis Ax2. The first reduction unit 41 reduces the rotation of the motor shaft of the motor 31 in speed when the worm wheel 52 is rotated.

The second reduction unit 42 includes an eccentric shaft 61 projecting from the worm wheel 52, an external gear 62 serving as a second gear, the stopper plate 63 serving as a restriction member, and an internal gear 64 serving as an internal gear. In the present embodiment, the eccentric shaft 61 is joined and integrated with the worm wheel 52. The eccentric shaft 61 and the worm wheel 52 form a first gear 69. The structure of the first gear 69 will be described later.

The components of the second reduction unit 42 will now be described.

Eccentric Shaft 61

As shown in FIGS. 3 and 4 , the eccentric shaft 61 projects from the worm wheel 52 along a rotation axis Ax3. The rotation axis Ax3 is parallel to the rotation axis Ax2 and separated from the rotation axis Ax2. Thus, the rotation axis Ax3, which is the center of the eccentric shaft 61, is offset from the rotation axis Ax2, which is the center of the worm wheel 52.

The eccentric shaft 61 includes an insertion hole 61 c extending from a distal end surface 61 b at the side closer to the internal gear 64. The center of the insertion hole 61 c coincides with the rotation axis Ax2. The rotation axis Ax3, which is the center of an outer circumferential surface 61 a of the eccentric shaft 61, is offset from the rotation axis Ax2. As will be described later, the output shaft 43 is inserted through the insertion hole 61 c of the eccentric shaft 61 in a relatively rotatable manner.

The eccentric shaft 61 is rotatable integrally with the worm wheel 52. The worm wheel 52, which rotates about the rotation axis Ax2, moves at least the outer circumferential surface 61 a of the eccentric shaft 61 in a circumferential direction about the rotation axis Ax2. Thus, the eccentric shaft 61 revolves about the rotation axis Ax2. The rotation axis Ax3 also moves (revolves) together with the eccentric shaft 61 in the circumferential direction about the rotation axis Ax2.

Stopper Plate 63

As shown in FIG. 3 , the stopper plate 63 is located between the worm wheel 52 and the external gear 62 in the direction of the rotation axis Ax2. The stopper plate 63, which is supported by the housing 36, supports the external gear 62.

As shown in FIGS. 5 and 6 , the stopper plate 63 includes a generally rhombic main body 63 a. The main body 63 a includes two corners opposing each other in the front-rear direction DY, and two corners opposing each other in the width direction DX.

Two arms 63 b extend toward opposite sides in the width direction DX from the middle of the main body 63 a with respect to the front-rear direction DY. One of the two arms 63 b includes a bifurcated branching portion 63 e.

Two recesses 63 c, or notches, are recessed inward in the front-rear direction DY at the middle part of the main body 63 a with respect to the width direction DX. A circular through hole 63 d extends through the central part of the main body 63 a.

As shown in FIG. 6 , the main body 63 a of the stopper plate 63 includes a protrusion 63 f The protrusion 63 f, which is located between one of the two recesses 63 c and one of the two arms 63 b, protrudes from the main body 63 a in a direction extending along the surface of the stopper plate 63. The protrusion 63 f of the stopper plate 63 facilitates visual recognition of the front and rear sides of the stopper plate 63.

In the example shown in FIG. 6 , the side of the stopper plate 63 facing the viewer is the front side. In this case, if the stopper plate 63 were to be accommodated in the housing 36 with the front and rear sides reversed, one would easily notice that the front and rear sides of the stopper plate 63 are reversed because of the different position of the protrusion 63 f.

Further, when assembling the gear device 32, a stack of stopper plates 63 may be prepared so that the stopper plates 63 are removed from the stack one at a time and coupled to a housing 36. In this case, the protrusions 63 f are arranged at the same position in the stack so that there will be no stopper plates 63 with reversed front and rear sides. A jig for stacking the stopper plates 63 in advance may be used so that interference will occur with the protrusion 63 f when a stopper plate 63 is set on the jig with front and rear sides reversed.

The protrusion 63 f is not particularly limited in shape and may be replaced by a recess. The protrusion 63 f may have any shape as long as it does not affect the performance required for the stopper plate 63.

As shown in FIGS. 5 and 6 , the arms 63 b of the stopper plate 63 are accommodated in two accommodation pits 36 a of the housing 36.

As shown in FIGS. 6 and 7 , the accommodation pits 36 a of the housing 36 are located in the inner wall of the housing 36 at the two ends in the width direction DX and outwardly recessed in the width direction DX. Each accommodation pit 36 a includes a peripheral wall 36 b and a bottom wall 36 c and is open upward in the up-down direction DZ and inward in the width direction DX.

One of the accommodation pits 36 a includes a projected wall 36 d projecting upward from the bottom wall 36 c. The projected wall 36 d is continuous with the corresponding peripheral wall 36 b. The projected wall 36 d includes a first projected wall 36 d 1, continuous with the peripheral wall 36 b, and a second projected wall 36 d 2, continuous with the first projected wall 36 d 1. The first projected wall 36 d 1 and the second projected wall 36 d 2 extend in the width direction DX.

As shown in FIG. 8 , the first projected wall 36 d 1 is greater in upward projecting height than the second projected wall 36 d 2 and greater in thickness than the arm 63 b. The projecting height of the second projected wall 36 d 2 is less than the thickness of the arm 63 b.

As shown in FIG. 7 , the distal portion, or upper end, of the first projected wall 36 d 1 includes sloped surfaces F1. More specifically, a view of the first projected wall 36 d 1 taken in the width direction DX will show the first projected wall 36 d 1 narrowed toward its tip by the sloped surfaces F1.

A view of the accommodation pit 36 a taken in the width direction DX will show that the middle of the peripheral wall 36 b of the accommodation pit 36 a in the up-down direction DZ includes inclined surfaces F2 inclined outward in the front-rear direction DY The inclined surfaces F2 results in the dimensions of the accommodation pit 36 a in the front-rear direction DY being greater at an upper portion than a middle portion in the up-down direction DZ. Thus, the opening is relatively large at the upper end of the accommodation pit 36 a.

As described above, the projecting height of the first projected wall 36 d 1 is greater than the thickness of the arm 63 b, the distal portion of the first projected wall 36 d 1 is narrowed toward its tip, and the opening at the upper end of the accommodation pit 36 a is relatively large. Thus, when accommodating the corresponding arm 63 b in the accommodation pit 36 a, the arm 63 b can be guided into the accommodation pit 36 a. More specifically, when accommodating the arm 63 b in the accommodation pit 36 a, even if the arm 63 b is slightly displaced relative to the accommodation pit 36 a, the first projected wall 36 d 1 and the peripheral wall 36 b of the accommodation pit 36 a will contact the arm 63 b and easily guide the arm 63 b to the correct position. Further, the projecting height of the second projected wall 36 d 2 is less than the thickness of the arm 63 b so that the second projected wall 36 d 2 does not overly interfere with the arm 63 b. This facilitates accommodation of the arm 63 b. Thus, the arm 63 b can be readily accommodated in the accommodation pit 36 a.

The projecting height of the first projected wall 36 d 1 is greater than the thickness of the arm 63 b. The distal portion of the first projected wall 36 d 1 is narrowed toward its tip. The opening at the upper end of the accommodation pit 36 a is relatively large. Such a structure functions as a guide of the accommodation pit 36 a.

To function as the guide, the accommodation pit 36 a does not have to satisfy all of the conditions described above, which are the projecting height of the first projected wall 36 d 1 being greater than the thickness of the arm 63 b, the distal portion of the first projected wall 36 d 1 being narrowed toward its tip, and the opening at the upper end of the accommodation pit 36 a being relatively large. The accommodation pit 36 a can function as the guide by satisfying one or more of these conditions. The guide may be omitted.

As shown in FIG. 8 , when the arm 63 b is accommodated in the accommodation pit 36 a, the first projected wall 36 d 1 and the second projected wall 36 d 2 are sandwiched by the bifurcated portion of the arm 63 b. The first projected wall 36 d 1 and the second projected wall 36 d 2 extend from the peripheral wall 36 b toward the branching portion 63 e of the arm 63 b.

When the arms 63 b are accommodated in the corresponding accommodation pits 36 a of the housing 36, linear movement of the stopper plate 63 is permitted in the width direction DX and restricted in the front-rear direction DY. More specifically, the arm 63 b contacts the peripheral wall 36 b of the accommodation pit 36 a, and the bifurcated portion of the arm 63 b contacts the first projected wall 36 d 1 and the second projected wall 36 d 2. This restricts movement of the arm 63 b in the front-rear direction DY.

When a rotating force is applied to the stopper plate 63, the force is received by the first projected wall 36 d 1 and the second projected wall 36 d 2 in addition to the peripheral wall 36 b in the accommodation pit 36 a. In this manner, the force applied by the arm 63 b is distributed to different parts of the accommodation pit 36 a. This improves the durability of the accommodation pit 36 a and the arm 63 b.

The branching portion 63 e is provided in only one of the arms 63 b but may be provided in both of the arms 63 b. In the same manner, the projected wall 36 d is provided in only one of the accommodation pits 36 a of the housing 36 but may be provided in both of the accommodation pits 36 a.

The projected wall 36 d may be omitted from the accommodation pit 36 a of the housing 36. In the same manner, the branching portion 63 e may be omitted from the arm 63 b of the stopper plate 63.

External Gear 62

As shown in FIGS. 3 and 4 , the external gear 62 includes a disc-shaped main body 62 a, the center of which is the rotation axis Ax3, and a flange 62 b, located at the lower side of the main body 62 a in the up-down direction DZ. The flange 62 b has a larger outer diameter than the main body 62 a. External teeth 62 c are formed on the outer circumference of the main body 62 a. The external teeth 62 c project from the main body 62 a in the radial direction with respect to the rotation axis Ax3 and are arranged at substantially equal intervals in the circumferential direction with respect to the rotation axis Ax3. An insertion hole 62 d extends through the central portion of the main body 62 a along the rotation axis Ax3.

As shown in FIGS. 9 and 13 , two engagement claws 62 e project downward in the up-down direction DZ from the flange 62 b of the external gear 62 at opposite sides of the rotation axis Ax3. The width W1 of each engagement claw 62 e in the width direction DX is slightly less than the width W2 of each recess 63 c of the stopper plate 63 in the same direction.

As shown in FIGS. 10 and 12 , the distance D1 between the engagement claws 62 e is greater than the distance D2 between the distal ends of the recesses 63 c in the stopper plate 63.

The external gear 62 is set on the stopper plate 63, and the engagement claws 62 e of the external gear 62 are received in the recesses 63 c of the stopper plate 63. As a result, the external gear 62 becomes linearly movable relative to the stopper plate 63 in the front-rear direction DY over a distance corresponding to the difference between the distances D1 and D2. Further, liner movement of the external gear 62 relative to the stopper plate 63 is restricted in the width direction DX.

FIG. 11 shows the external gear 62 with its lower surface facing the upper side. That is, FIG. 11 shows the external gear 62 reversed upside down.

As shown in FIGS. 11 and 13 , the engagement claws 62 e are posts having a rectangular cross section and projecting in a thickness direction of the external gear 62. Each engagement claw 62 e includes two opposing first side walls 65 and two opposing second side walls 66. The opposing direction of the first side walls 65 corresponds to the front-rear direction DY, which is the linear movement direction of the external gear 62.

As shown in FIGS. 11 and 13 , each engagement claw 62 e includes a distal end. A beveled surface F3 extends along the periphery of the distal end. More specifically, the four edges at the distal end of the engagement claw 62 e include beveled surfaces F3. Further, the four sides of the engagement claw 62 e extending from the basal end to the four edges of the distal end also include beveled surfaces F4. In this manner, each engagement claw 62 e includes the beveled surfaces F3 and F4. Thus, when grease is applied between the engagement claws 62 e of the external gear 62 and the recesses 63 c of the stopper plate 63, the grease is held on the beveled surfaces F3 and F4. Since grease is held on the beveled surfaces F3 and F4, the grease will not escape from between the engagement claws 62 e and the recesses 63 c when the external gear 62 moves linearly. Thus, the beveled surfaces F3 and F4 of the engagement claws 62 e function as grease sumps. All of the edges and sides of the engagement claws 62 e do not have to include the beveled surfaces F3 and F4 as described above, and only some of these portions may include the beveled surfaces F3 and F4.

Among the four edges of the distal end of each engagement claw 62 e, it is preferred that the beveled surfaces F3 and F4 be included in the edge continuous with the first side wall 65 located at the radially outward side of the external gear 62 and the sides continuous with the second side walls 66 in the first side wall 65 that is located at the radially outward side of the external gear 62. This allows grease sumps to be formed in the portions of the engagement claws 62 e facing the movement direction of the external gear 62 when the external gear 62 moves linearly in the radial direction. This limits the grease that escapes outward in the radial direction. The beveled surfaces F4 provided at the locations described above form grease sumps over a relatively large range on each engagement claw 62 e.

The grease sumps do not have be shaped as the beveled surfaces F3 and F4. Depressions may be formed in the first side walls 65 and the second side walls 66 of the engagement claws 62 e to hold grease. The edges of the recesses 63 c in the stopper plate 63 may include beveled surfaces to form gaps extending to the corresponding engagement claws 62 e so that the gaps function as grease sumps.

The grease sumps may be omitted.

The engagement claws 62 e of the external gear 62 do not have to be accommodated in the recesses 63 c of the stopper plate 63 like in the present embodiment. The external gear 62 may include recesses, and the stopper plate 63 may include engagement claws. In this case, the engagement claws of the stopper plate 63 are accommodated in the recesses of the external gear 62. In this manner, the structures fitted together to engage the external gear 62 and the stopper plate 63 may be reversed.

Internal Gear 64

As shown in FIGS. 3 and 4 , the internal gear 64 includes a disc-shaped main body 64 a and a flange 64 b located at the lower side of the main body 64 a in the up-down direction DZ. The flange 64 b has a larger diameter than the main body 64 a. The inner circumference of the flange 64 b includes inner teeth 64 c.

The inner teeth 64 c project from the flange 64 b inward in the radial direction with respect to the rotation axis Ax2 and are arranged at substantially equal intervals in the circumferential direction with respect to the rotation axis Ax2. There are more inner teeth 64 c than the external teeth 62 c. For example, the number of the inner teeth 64 c is greater by one or two than the number of the external teeth 62 c.

An insertion hole 64 d extends through a central portion of the main body 64 a of the internal gear 64. The center of the insertion hole 64 d coincides with the rotation axis Ax2. The output shaft 43 is inserted through the insertion hole 64 d. The internal gear 64 is rotated integrally with the output shaft 43.

The stopper plate 63, the external gear 62, the internal gear 64, the output shaft 43, the eccentric shaft 61, and the worm wheel 52 are not particularly limited in material although it is preferably that the material be metal to obtain superior mechanical strength and heat resistance.

Assembled Mechanism of Second Reduction Unit 42

As shown in FIG. 3 , the urging member 39 is arranged in the housing 36. Further, the first gear 69, which includes the eccentric shaft 61 and the worm wheel 52, are accommodated in the housing 36. Then, the stopper plate 63, which serves as a restriction member, is arranged on the first gear 69. The arms 63 b of the stopper plate 63 are arranged in the corresponding accommodation pits 36 a of the housing 36, and the eccentric shaft 61 is inserted through the through hole 63 d of the stopper plate 63. The external gear 62, which serves as a second gear, is arranged on the stopper plate 63. The engagement claws 62 e of the external gear 62 are arranged in the recesses 63 c of the stopper plate 63. The output shaft 43 is inserted through the insertion hole 62 d of the external gear 62 and the insertion hole 61 c of the eccentric shaft 61 to couple the internal gear 64. At least one of the inner teeth 64 c of the internal gear 64 is meshed with at least one of the external teeth 62 c of the external gear 62. Then, screws 38 are used to attach the cover 37 to the housing 36. The cover 37 includes an insertion hole 37 a. The pinion 29 a of the output shaft 43 extends through the insertion hole 37 a and out of the housing 36.

The urging member 39 may be omitted. Thus, the second reduction unit 42 may be assembled without the urging member 39. The urging member 39 does not have to be a Belleville spring and may be a rubber member or a spring.

As shown in FIGS. 3 and 4 , the cover 37 includes a curved groove 37 b projecting downward in the up-down direction DZ around the insertion hole 37 a. The groove 37 b is annular and formed around the insertion hole 37 a through a beading process.

As shown in FIG. 4 , in a view taken in the front-rear direction DY, the groove 37 b is located between the central portion and outer circumferential portion of the internal gear 64, closer to the outer circumferential portion, in the width direction DX. In the same manner, in a view taken in the width direction DX, the groove 37 b is located between the central portion and outer circumferential portion of the internal gear 64, closer to the outer circumferential portion, in the front-rear direction DY. The groove 37 b includes a bottom point P that is in contact with the internal gear 64.

The groove 37 b of the cover 37 increases the strength of the cover 37 in the thickness direction. This limits deformation of the cover 37 in the thickness direction. The groove 37 b functions as a deformation resistance portion.

The groove 37 b is formed through a beading process. Thus, the deformation resistance portion can be formed through a simple process.

As shown in FIG. 4 , the urging member 39, the eccentric shaft 61, the worm wheel 52, the stopper plate 63, the external gear 62, the internal gear 64, and the like are arranged in the housing 36. As described above, rotation of the worm wheel 52 eccentrically offsets the external gear 62 and reduces the speed of the rotation of the internal gear 64. The repulsion force of the urging member 39 in the housing 36 produces an urging force directed upward with respect to the up-down direction DZ. Multiple components are moved in cooperation in the housing 36. This increases stress that acts in the upward direction. The urging force and stress results in a tendency to upwardly move the outer circumferential portion of the internal gear 64 and thereby deform the cover 37. In the present embodiment, the groove 37 b is located between the central portion and outer circumferential portion of the internal gear 64, closer to the outer circumferential portion, in the width direction DX. This limits deformation in the thickness direction of the cover 37. Further, the bottom point P of the groove 37 b contacts the internal gear 64. This limits deformation in the thickness direction of the cover 37.

The deformation resistance portion does not have to be shaped like the groove 37 b. For example, the thickness of the cover 37 may be locally increased to resist deformation. Alternatively, reinforcements may be locally attached to the cover 37 in order to resist deformation.

Further, the groove 37 b does not have to be located between the central portion and outer circumferential portion of the internal gear 64, closer to the outer circumferential portion, in the width direction DX. The groove 37 b may be located at a middle point between the central portion and outer circumferential portion of the internal gear 64 in the width direction DX. Alternatively, the groove 37 b may be located closer to the central portion. The bottom point P of the groove 37 b in the cover 37 may be located proximate to the internal gear 64 instead of contacting it.

Moving Mechanism of Second Reduction Unit 42

When the worm wheel 52 of the first gear 69 is rotated about the rotation axis Ax2, the eccentric shaft 61 revolves about the rotation axis Ax2. The stopper plate 63 does not allow the external gear 62 to rotate together with the eccentric shaft 61. The external gear 62 is moved in the circumferential direction with respect to the rotation axis Ax2 along a circular path, the radius of which is the distance between the rotation axis Ax2 and the rotation axis Ax3. Thus, the external gear 62 revolves about the rotation axis Ax2.

When the external gear 62 revolves, the external teeth 62 c of the external gear 62 moves in the circumferential direction with respect to the rotation axis Ax2. The number of the inner teeth 64 c of the internal gear 64 is greater than the number of the external teeth 62 c. Thus, as the external gear 62 revolves, the portion where the inner teeth 64 c is meshed with the external teeth 62 c moves about the rotation axis Ax2. Movement of the portion where the inner teeth 64 c is meshed with the external teeth 62 c about the rotation axis Ax2 revolves the internal gear 64 about the rotation axis Ax2. When the external gear 62 completes a single revolution, the internal gear 64 is rotated by an amount corresponding to the movement of the portion where the inner teeth 64 c is meshed with the external teeth 62 c. This reduces the rotation speed of the internal gear 64 from the rotation speed of the worm wheel 52. Thus, the internal gear 64 is rotated at a lower speed than the first gear 69. In this manner, the second reduction unit 42 reduces speed through a taumel mechanism.

Operation and Advantages of Present Embodiment

-   -   (1) The gear device 32 includes the housing 36 and the cover 37.         The housing 36 accommodates the first gear 69, the external gear         62 (second gear), and the internal gear 64 (third gear). The         cover 37 is attached to the housing 36. The cover 37 includes         the groove 37 b (deformation resistance portion). This limits         deformation of the cover 37 in the thickness direction.     -   (2) When viewing the cover 37 in the axial direction of the         third gear 64, the groove 37 b (deformation resistance portion)         is located between the central portion and outer circumferential         portion of the internal gear 64 (third gear), closer to the         outer circumferential portion. This limits deformation of the         cover 37 in the thickness direction.     -   (3) The groove 37 b (deformation resistance portion) is in         contact with the internal gear 64 (third gear). This further         limits deformation of the cover 37 in the thickness direction.     -   (4) The groove 37 b (deformation resistance portion) is formed         through a beading process. Thus, the groove 37 b (deformation         resistance portion) can be easily formed.

OTHER EMBODIMENTS

The above embodiment may be modified as described below. The above embodiment and the following modifications can be combined as long as there is no technical contradiction.

The second gear does not have to be the external gear 62, and the third gear does not have to be the internal gear 64. The second gear may be an inner gear, and the third gear may be an external gear.

In the above embodiment, the gear device 32 includes the rotation transmission mechanism 35, the housing 36, and the cover 37. Further, the rotation transmission mechanism 35 includes the first reduction unit 41, the second reduction unit 42, and the output shaft 43. However, there is no limitation to such structures. The first reduction unit 41 may be omitted from the rotation transmission mechanism 35. In this case, the second reduction unit 42 of the rotation transmission mechanism 35 may directly reduce the speed of the rotation produced by the motor 31. Further, a typical external gear may be used instead of the worm wheel 52 as a gear portion.

The gear device 32 does not have to be used in a seat lifter that raises and lowers the seat 11 of a vehicle. The gear device 32 may be any gear device that includes a taumel mechanism. A gear device including a taumel mechanism is, for example, a gear device that slides a seat in front and rear directions or a gear device that reclines a seat.

Various changes in form and details may be made to the examples above without departing from the spirit and scope of the claims and their equivalents. The examples are for the sake of description only, and not for purposes of limitation. Descriptions of features in each example are to be considered as being applicable to similar features or aspects in other examples. Suitable results may be achieved if sequences are performed in a different order, and/or if components in a described system, architecture, device, or circuit are combined differently, and/or replaced or supplemented by other components or their equivalents. The scope of the disclosure is not defined by the detailed description, but by the claims and their equivalents. All variations within the scope of the claims and their equivalents are included in the disclosure. 

1. A gear device, comprising: a first gear; a second gear offset eccentrically when the first gear is rotated; a third gear rotated at a speed slower than a rotation speed of the first gear when the second gear is offset eccentrically; a housing that accommodates the first gear, the second gear, and the third gear; and a cover attached to the housing, wherein the cover includes a deformation resistance portion that limits deformation of the cover in an axial direction of the third gear.
 2. The gear device according to claim 1, wherein when viewing the cover in the axial direction of the third gear, the deformation resistance portion is located between a central portion and outer circumferential portion of the third gear, closer to the outer circumferential portion.
 3. The gear device according to claim 1, wherein the deformation resistance portion is in contact with the third gear.
 4. The gear device according to claim 1, wherein the deformation resistance portion is formed through a beading process.
 5. The gear device according to claim 2, wherein the deformation resistance portion is formed through a beading process.
 6. The gear device according to claim 3, wherein the deformation resistance portion is formed through a beading process. 